1. Technical Field
The present invention relates to an oscillator circuit, an oscillator, an electronic apparatus, and an activation method of the oscillator circuit.
2. Related Art
Voltage Controlled Oscillators (VCOs) that can change oscillation frequencies in response to control voltages have been widely known and used for various uses. Voltage Controlled X'tal Oscillators (VCXOs) using crystal oscillators having high frequency stability and Voltage Controlled SAW Oscillators (VCSOs) using Surface Acoustic Wave (SAW) resonators, etc. are used for various uses. The VCXO has high frequency stability and the VCSO has a high oscillation frequency, and thus, these oscillators are selectively used for intended uses. In these oscillators, a variable capacitance element is connected to one end of the resonator such as the crystal oscillator or the SAW resonator, and thereby, the oscillator may be oscillated at the frequency in response to the capacitance value of the variable capacitance element between the resonance frequency and the antiresonance frequency of the resonator. In the case where the difference between the resonance frequency and the antiresonance frequency is smaller, i.e., the frequency variable range as the VCO is narrower, an inductance element (elongated coil) is inserted in series in the resonator for the purpose of widening the frequency variable range as the VCO. However, by insertion of the elongated coil, an LC oscillation mode by the inductance L of the elongated coil and capacitance of the circuit (capacitance of the variable capacitance element or the like) C also exists in addition to the original oscillation mode, and the LC oscillation may be selected depending on various conditions at activation of oscillation and oscillation at the original frequency may not be performed.
In order to solve the problem, Patent Document 1 (JP-A-2010-4322) has proposed a method of preventing abnormal oscillation by setting load capacitance of the crystal oscillator to load capacitance that may hardly cause abnormal oscillation using the variable capacitance circuit only at activation of oscillation.
However, in the method of Patent Document 1, not only the setting of the variable capacitance circuit is necessary but also a larger capacitance value is necessary depending on the inductance value of the elongated coil, and the mounting area and the cost may be significantly increased.